Recently, transmission schemes for higher transmission rates in wireless communication systems are being researched and standardized. In order to have such a high transmission rate in wireless LAN systems as well, a structure having a transmission rate of a maximum of 600 Mbps has been standardized, to which a MIMO system having multiple input/output in IEEE 802.11 TGn has been applied. There has been discussion in IEEE 802.11 VHTSG regarding a system having a maximum transmission rate of 1 Gbps at MAC SAP, and the task group of IEEE 802.11 TGac/TGad has been established accordingly. In order to maintain frequency efficiency while satisfying such a high transmission rate, the AP and STA must support more streams than four, which are supported by TGn, requiring a large number of antennas. In STA's terms, it is difficult to support a large number of antennas, considering the complexity or power consumption of the STA. Therefore, multi-user MIMO is being considered, according to which the AP simultaneously transmits to multiple STAs.
FIG. 1 illustrates an example of transmitting data to different STAs in a multi-user MIMO environment.
As illustrated in FIG. 1, according to multi-user MIMO, multiple STAs simultaneously transmit respective pieces of data, and the length of data transmitted by each STA varies depending on the supported service type or transmission rate. That is, each STA transmits a different length of data, as illustrated in FIG. 1, and in order to establish a transmission rule, data transmission needs to be performed as illustrated in (a) or (b).
Specifically, the starting point of each STA is brought into coincidence as in the case of (a) of FIG. 1. That is, each STA starts transmitting data at the same time of t0. Then, the ending point of each STA may fall on different times, such as t3 and t4, depending on the data type, service type, and transmission rate.
Alternatively, the ending point of each STA is brought into coincidence as in the case of (b) of FIG. 1. That is, each STA has the same ending point of t10, although they have different transmission starting points of t0, t1, and t2.
However, there has been proposed no method for transmitting, by each STA, data based on VHT wireless LAN system standards. Therefore, there is a need for a method for transmitting, by STAs, data most efficiently.